An action plane is understood to mean a plane wherein the measuring beam meets the object and is influenced by the movement of the object and the device relative to each other. The action plane may be the plane of the device window or a plane near this window. The laser self-mixing effect is understood to mean the phenomenon that radiation emitted by a diode laser and reflected back in the laser cavity causes changes in operation of the laser cavity, which changes are due to interference of the reflected radiation re-entering the cavity with the optical wave generated in the laser cavity. The optical input device uses the self-mixing effect in combination with the Doppler effect. The latter effect is the phenomenon that a beam of radiation reflected by an object undergoes a frequency shift called Doppler shift if the object moves in the propagation direction of the beam. If the reflected beam re-enters the cavity of the laser, which emits the beam, changes in operation of the laser cavity will occur, which changes are determined by the movement of the object. Thus, by measuring these changes, for example by measuring the changes in the beam emitted at the front side or the rear side of the laser or by measuring the electrical characteristic of the laser cavity, the movement of the object can be determined. This movement represents data to be inputted in an apparatus of which the input device forms part.
Such an input device is disclosed in PCT application WO 02/37410, which describes the principle of operation of the input device, a number of embodiments and possible applications of the device and is incorporated herein by reference.
It has been found that, although the principle of the device has been proven in practice, under certain circumstances it does not function in an optimum way.